An antenna array is a collection of antenna elements which are collectively used to transmit or to receive one or more wireless signals. An antenna array can be used in an antenna arrangement to achieve a variable antenna pattern. The antenna pattern of an antenna arrangement describes the gain of the antenna arrangement as a function of azimuth and elevation.
An antenna arrangement which has an antenna pattern with a significantly larger gain in one direction compared to other directions is referred to as having a main lobe, or main beam, in the direction with high gain. The width of this main beam is herein referred to as the beamwidth of the antenna arrangement. The beamwidth of an antenna arrangement in an elevation direction is herein referred to as the elevation beamwidth of the antenna arrangement.
All antenna arrangements discussed herein are assumed to be reciprocal, meaning that the antenna pattern of an antenna arrangement is substantially equal for transmission and reception of wireless signals.
Antenna arrangements used, e.g., by base stations in cellular communication networks can implement multiple antenna elements in the elevation domain to achieve a narrow elevation beamwidth. Furthermore, if the output power of individual antenna elements can be varied, it becomes possible to dynamically change this elevation beamwidth by changing the output power of different antenna elements.
At least partly towards this end, the individual antenna elements in some antenna arrays have separate radio units and thus also separate amplifiers connected to the individual antenna elements. That is, each antenna element in the array has its own radio unit and amplifier. This enables altering an elevation beamwidth of the antenna array by reducing the output power of one or more of the antenna elements, which can be achieved by simply lowering the output power of the respective individual antenna element amplifiers.
Other antenna arrays make use of active antenna elements, which active antenna elements comprise respective steerable amplifiers which control the individual output powers of the active antenna elements. By controlling output powers of the different antenna elements in this way, the antenna pattern of the antenna arrangement can be varied.
Of course, attenuators can be used instead of antenna element amplifiers with a similar effect.
Controlling individual antenna element output powers by respective antenna element amplifiers, or attenuators, is herein referred to as amplitude tapering of the antenna arrangement.
A negative effect on the power efficiency and on the total output power of the antenna arrangement can be the result when using antenna arrangements with a plurality of individual amplifiers to control, e.g., elevation beamwidth and/or the direction of a main lobe, by varying individual antenna element amplification or attenuation factors. The reason is that some power amplifiers must reduce their output power in order to change beamwidth, and is thus not contributing maximally to output power. The same happens when individual antenna element attenuators are used to control elevation beamwidth, since output power is lost in the attenuation.
Thus, there is a need for an antenna arrangement with variable antenna pattern and improved power efficiency and total output power.